1. Field of the Invention
This invention relates to a superconducting electromagnet apparatus, and more particularly to a superconducting electromagnet apparatus for use in a magnetic resonance imaging system.
2. Description of the Prior Art
In general, an electromagnet for a magnetic resonance imaging system needs to generate a highly intense and homogeneous magnetic field in an imaging space within the opening of the electromagnet. When generating such a magnetic field, a superconducting electromagnet apparatus in the prior art has a problem in that the magnetic field leaks outside the electromagnet to adversely affect peripheral equipment. It is therefore desirable that the magnetic field leakage be reduced by subjecting the electromagnet to magnetic shielding.
An expedient for the magnetic shielding of the electromagnet is a method in which the electromagnet is surrounded with a magnetic substance. This method, however, has the disadvantage that the weight of the apparatus increases so that high load bearing strength is required at the place of installation. For this reason, a method is employed in which a coil for generating a magnetic field opposite in polarity to that of the main coil is disposed around the main coil.
FIG. 1 is a sectional view showing the construction of an electromagnet which is used in the prior-art superconducting electromagnet apparatus stated above. A superconducting main coil (hereinbelow, simply termed "main coil") 1 and a superconducting shield coil (hereinbelow, simply termed "shield coil") 2, which is disposed around the main coil 1 and which reduces external magnetic field leakage generated by the main coil 1, are housed in an inner vessel 3 and are cooled to and held at a cryogenic temperature. The inner vessel 3 is further enclosed within a vacuum vessel 4 for the purpose of vacuum thermal insulation. A thermally-insulating shield 5 is interposed between the inner vessel 3 and the vacuum vessel 4.
FIG. 2 is an electric connection diagram of the prior-art superconducting electromagnet apparatus. The main coil 1 and the shield coil 2 are connected in series, and a persistent current switch 6 is connected in parallel with the series connection assembly. The resulting parallel connection assembly is connected to a magnetizing power source 10 through current leads 7, detachable current leads 8 and current leads 9. The persistent current switch 6 is combined with a heater 11, which is connected to a heater power source 14 through current leads 12, detachable current leads 8 and current leads 13.
In such a superconducting electromagnet apparatus, the shield coil 2 establishes a homogeneous resultant magnetic field in the internal operating space of the electromagnet in superposition with a magnetic field generated by the main coil 1. Shield coil 2 also generates an external magnetic field opposite in polarity to the external magnetic field leakage generated by the main coil 1, thereby to reduce the resultant magnetic field combined with the magnetic field of the main coil 1.
In magnetizing the electromagnet, the detachable current leads 8 are attached to connect the magnetizing power source 10 and the heater power source 14. The heater 11 is energized by the heater power source 14 to open the persistent current switch 6, whereby the main coil 1 and shield coil 2 connected in series are supplied with current from the magnetizing power source 10. After the current has reached a predetermined value, the persistent current switch 6 is closed. Then, the main coil 1 and the shield coil 2 form a series closed circuit, so that a persistent current operation is performed.
With the prior-art superconducting electromagnet apparatus shown in FIG. 2, since the main coil 1 and the shield coil 2 are connected in series, the conduction currents thereof are equal, and the homogeneous resultant magnetic field must to be established in the internal operating space of the electromaget with the identical current. Therefore, such an apparatus has problems in that the size of the conductors and the shapes of the coils are limited making it uneconomical.
In order to relieve this problem, a superconducting electromagnet apparatus as shown in FIG. 3 has been employed. In this superconducting electromagnet apparatus, a persistent current switch 6a is connected in parallel with the main coil 1, and it is combined with a heater 11a. Further, the main coil 1 is also connected to a magnetizing power source 10a through current leads 7a, detachable current leads 8 and current leads 9a, while the heater 11a is connected to a heater power source 14a through current leads 12a, detachable current leads 8 and current leads 13a. Likewise, a persistent current switch 6b is connected in parallel with the shield coil 2, and it is combined with a heater 11b. Further, the shield coil 2 is also connected to a magnetizing power source 10b through current leads 7b, detachable current leads 8 and current leads 9b, while the heater 11b is connected to a heater power source 14b through current leads 12b, detachable current leads 8 and current leads 13b.
With the superconducting electromagnet apparatus in FIG. 3, the main coil 1 and the shield coil 2 are electrically independent of each other and have independent magnetizing circuits, so that they can be operated with currents different from each other. This superconducting electromagnet apparatus, however, has two problems. Since current leads are required for the two circuits for the main coil and for the shield coil, the number of internal circuits for the detachable current leads 8 increases, increasing the size of the apparatus. Also, two magnetizing power sources are required instead of the single source as in FIG. 2.
As thus far described, the prior-art super-conducting electromagnet apparatus has had the problems of being uneconomical due to the limitations on the sizes of conductors and the shapes of the coils or the problems of being too large and requiring two magnetizing power sources.